In the event of a national emergency, a series of seven different alert Conditions (LERTCONs) can be called. The 7 LERTCONs are broken down into 5 Defense Conditions (DEFCONs) and 2 Emergency Conditions (EMERGCONs).
The furthest we have ever gotten to on the LERTCON ladder is DEFCON 2, during the cuban missile crisis in 1965.